The present invention relates generally to a cutter holder attached to a spindle of a machine tool for supporting a cutting tool rotated by the spindle. More particularly, the invention is concerned with such a holder for a rotary cutting tool, which causes minimum decrease in accuracy of machining by the tool due to a positioning error of the holder with respect to the machine spindle.
Various tool holders have been used for mounting drills, milling cutters, reamers, boring bars and other rotary cutting tools on spindles of machine tools such as drilling, milling and boring machines, and machining centers which are capable of automatically performing multiple kinds of cutting operations. Such a tool holder supports a tool at its one end portion, and is adapted to be removably fixed to the machine spindle at the other end portion. An example of the known tool holders is illustrated in FIG. 6.
In the figure, a tool holder 2 is mounted on a spindle 1 of a machining center. The tool holder 2 carries a fine-boring bar 4 mounted at one end thereof, and has a pull stud 5 fixed to the other end. The holder 2 has a taper shank 6 adjacent to the end on the side of the pull stud 5. The tool holder 2 is automatically mounted on the spindle 1 by means of a tool changer arm (not shown) such that the taper shank 6 fits in a mating tapered bore 7 formed in the spindle 1. With the shank 6 inserted in the spindle bore 7, the pull stud 5 engages a drawing-rod or draw bar (not shown) incorporated in the spindle 1, and the tool holder 2 is pulled by the draw bar inwardly of the spindle bore, whereby the tapered surface of the shank 6 is brought into close contact with the inner tapered surface 7 of the spindle 1. Thus, the tool holder 2 is secured to the spindle 1 with a high positioning accuracy.
However, the tapered surface of the shank 6 and the mating tapered surface 7 of the spindle are liable to wear due to frequent mounting and dismounting of tool holders for a variety of tools for the machining center. Further, these tapered surfaces are subject to cutting chips, dust and dirt, and other foreign matters. As a result, the tapered shank 6 does not always make a sufficiently snug fit in the tapered bore 7. Thus, it has been difficult or impossible to avoid a positioning error of the tool holder 2 due to misalignment or inclination of its centerline with respect to the centerline of the spindle 2. This positioning error or misalignment of the tool holder 2 results in an increase in run-out of the boring bar 4 at the free end of the holder 2, and consequently leads to machining errors such as an oversize of a bore diameter to be bored by the boring bar 4. That is, the positioning error of the tool holder 2 lowers the accuracy of machining with the tool 4.
The above indicated positioning error of a tool holder will give rise to serious trouble, particularly in fine-boring, reaming and other machining operations which require a relatively high accuracy. For this reason, such a high-accuracy machining operation has been performed in many cases on a special-purpose or single-purpose machine, and not on a machining center which utilizes a variety of cutting tools.